This invention relates to a method of and an apparatus for composing an approximate sinusoidal waveform by utilizing a squaring operation and a coordinate conversion process.
It is generally well known that any type of waveform can be obtained by such a method wherein the digital words indicative of a waveform are memorized in a memory device such as a read only memory (ROM) and read out by accessing its addresses by an input variable. This method is called a table look-up method since it is similar to a method of reading a function table. However, it has such disadvantages that a large number of read only memories are required to reduce quantization noise and therefore its reading speed is comparatively low. For example, if the number of address bits is increased by one to reduce the quantization noise to one half, then the table scale or data to be memorized is doubled compared with the original, thus resulting in enlargement of the hardware. When the data and the addresses are both expressed by twelve bits, the memory capacity should be as large as approximately 48 K bits (K = 1024). In general, to avoid such increase of memory capacity, the so called interpolation process may be utilized. In this process, the data for the points between respective adajacent addresses are calculated through an interpolation operation. But to perform such an interpolation process, multiplication circuits and other circuits associated therewith will necessarily be required, and therefore simplification of the hardware is almost impossible when considering the total hardware so long as the interpolation process is employed.